Organic electroluminescent devices have a simpler structure, various processing advantages, higher luminance, superior viewing angle, quicker response rate, and a lower operating voltage than other flat panel display devices such as liquid crystal displays (LCDs), plasma display panels (PDPs), field emission displays (FEDs), etc., and thus many attempts are being made to use them as a light source of flat panel displays such as wall-mountable TVs and so on or of the backlight units of displays, illuminators, advertisement boards and the like.
Typically, when direct-current voltage is applied to an organic electroluminescent device, holes injected from an anode and electrons injected from a cathode recombine to form electron-hole pairs, namely, excitons, after which the excitons return to a stable ground state and the corresponding energy is transferred to a light-emitting material and is thereby converted into light.
In the effort to increase efficiency and stability of an organic electroluminescent device, an organic electroluminescent device operating at low voltage was reported to be made by forming a tandem thin organic film between two opposite electrodes by C. W. Tang et al. in Eastman Kodak (C. W. Tang, S. A. Vanslyke, Applied Physics Letters, vol. 51, pp. 913, 1987), and thorough research into organic materials for organic electroluminescent devices with multilayered thin-film structures is ongoing. The lifetime of such a tandem organic electroluminescent device is closely related to the stability of the thin film and the material. For example, when the thermal stability of the material is lowered, the material may crystallize at high temperature or the operating temperature, undesirably shortening the lifetime of the device.
A variety of known compounds function as the conventional host materials of organic electroluminescent devices. These include triazine-based compounds, oxadiazole-based compounds, benzimidazole-based compounds, phenyl pyridine-based compounds, and silicon-based compounds. However, such compounds are problematic because superior efficiency characteristics cannot be achieved in the organic electroluminescent devices, and host materials able to exhibit superior characteristics in blue phosphorescent devices are considerably limited. Hence, the development of novel compounds to solve such problems is required.
As a novel host material, a novel phosphine oxide based compound has been reported. With this compound, however, it is difficult to attain high efficiency.
Korean Patent Publication No. 10-2006-0109524 discloses an arylphosphine oxide-based compound, an arylphosphine sulfide-based compound or an arylphosphine selenide-based compound and an organic electroluminescent device using the same, but is problematic because high efficiency cannot be obtained in a pure blue phosphorescent device.
Applied Physics Letter (Appl. Phys. Lett. 92, 083308, 2008) discloses a blue phosphorescent device using a phosphine oxide compound having a fluorene structure, but the quantum efficiency of the device is only about 9%, undesirably resulting in low device efficiency.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the related art and the present invention is intended to develop a novel host material, namely, a phosphine oxide compound having a carbazole structure, and apply it as the host material of an organic electroluminescent device.